It is known from U.S. Pat. No. 4,251,253 to form a blow-molding parison by dropping a glob of molten glass into the cup-like main mold body of the parison mold. Then a split mold middle piece and a neck mold are closed on the upper end of the main mold body. A plunger is then thrust down through the neck mold and the glass is pressed against the inner walls of a mold cavity of the parison mold formed by the main mold body, mold middle piece, and neck mold. The volume of the interior of the main mold body is constant. The gob volume may not be greater than said constant volume. Otherwise glass would overflow, get between the parts of the parison mold and make a mess of the equipment, or remain on the finished parison as enlarged seams or "wings", causing a product reject. Especially the bridging of the space between the neck diameter to the uppermost or loading diameter of the upper opening of the main mold body is critical. The resulting parison tends to be too thick in the shoulder region, making formation of a bottle of uniform wall thickness impossible.
In U.S. Pat. No. 4,336,050 the parison mold is provided with a piston and various complex structure that must be operated in a particular sequence. In addition many parts of the machine are kept in contact with the molten glass for quite some time, so they wear out and degrade rapidly.